Modified polymeric flocculants with improved performance characteristics

ABSTRACT

Methods for the preparation of high molecular weight cationic poly(meth) acrylamide/quaternary ammonium salt copolymers are disclosed wherein after at least about 50% of the monomers have been polymerized, a cross-linking agent is then added to the reaction mixture on a continuous basis and in the absence of any feed of chain transfer agent. Cationic copolymers so produced exhibit improved floc formation properties and are useful as flocculants and dewatering aids in aqueous systems that include an oily sludge or the like.

FIELD OF THE INVENTION

[0001] The present invention is directed to materials and methods forthe preparation of high molecular weight, acrylic-based, water-solublepolymers. More particularly, the present invention relates to certaincationic, acrylamide-based polymers that are prepared by adding astructural modifier after about 50% conversion of the monomers hasoccurred, and their use as flocculants and dewatering agents.

BACKGROUND OF THE INVENTION

[0002] The problems associated with the separation of suspended solidsfrom industrial process waters are well known to those skilled in theart. For example, see the background of the invention section of U.S.Pat. No. 3,601,039.

[0003] One such means of enhancing the settling or filtration ofsuspended solids from aqueous media is by chemical treatment. Inparticular, the chemical treatment process used to enhance thedewatering of activated sludge, which is typically anionic in nature, isdescribed by the addition of a cationic flocculant to the sludge whileagitating the sludge. The flocs that form are then subjected to adewatering operation. This treatment process consists of neutralizingthe negative electric charge of the sludge by the addition of apositively charged flocculent, the subsequent formation of small flocs,followed by the agglomeration of the small flocs into larger flocs thatsettle out of solution prior to the dewatering operation. For example,the use of polyamines as cationic flocculants for thickening anddewatering operations is taught in U.S. Pat. No. 3,975,266.

[0004] One objective of improving the flocculation process is to attainbetter dewatering performance by the formation of larger, sturdierflocs. U.S. Pat. No. 3,235,490 teaches the use of various chemicaladditives including certain polymers of acrylamide cross-linked withN,N′-methylenebisacrylamide. The polymers are claimed to flocculate thesuspended solids in an aqueous media by capture of fine particles withinthe structured polymer network. However, '490 teaches that saidcopolymers are only effective as cationic flocculants under acidic orneutral conditions.

[0005] U.S. Pat. No. 3,698,037 teaches a method of releasing water fromactivated sludge utilizing cross-linked, cationic emulsion copolymers.

[0006] U.S. Pat. No. 4,705,640 teaches that the flocculation performanceof certain cross-linked, cationic acrylamide dispersion copolymers isimproved by subjecting the polymer solution to mechanical shear prior toapplication.

[0007] U.S. Pat. No. 6,294,622 B1 teaches the process for thepreparation of certain cross-linked, cationic acrylamide emulsioncopolymers that exhibit improved flocculation performance without theuse of mechanical shear. The process described for preparing thecationic acrylamide copolymers comprises polymerizing the monomers whileadding the cross-linking agent (chain-branching agent) continuously tothe mixture throughout the course of the reaction. Furthermore, '622teaches that said addition of the cross-linking agent must begin beforeabout 50%, preferably about 25% of the monomer conversion is achieved.

[0008] The prior processes as exemplified by U.S. Pat. No. 6,294,622,exhibit the potential to produce a polymer with a higher degree ofcross-linking resulting from the continuous addition of a cross-linkingagent over the extent of the entire polymerization reaction.

[0009] Higher cross-linking may result in the reduction in the netcationic charge available for neutralization of the negative charge onthe substrate by trapping or shielding a portion of the charge withinthe three-dimensional polymer network. Moreover, the process shows afurther shortcoming in that, although the polymers formed possess alarger molecular weight relative to their linear counterparts, their netsize, or measure of true hydrodynamic volume, is compromised. Theinterlocking nature of the extensively bound polymer network preventsfull dispersion or extension into solution, particularly within thelimited time of the application.

[0010] WO 02/02662 A1 is directed to structurally modified water-solublepolymers made via polymerization of an aqueous solution of monomersunder free radical polymerization conditions. At least one structuralmodifier is added to the polymer solution after at least 30%polymerization of the monomers has occurred. The so-called structuralmodifiers can comprise either a chain transfer agent and/or across-linking agent. As is apparent from Ex. 7 of this publication, whena cross-linking agent is added alone in the later stages of thereaction, as the only structural modifier, it is always shot fed (i.e.,added all at once in a single treatment). However, as shown in Tables 29and 25, when the cross-linking agent is fed over time, it is always fedin combination with a chain transfer agent (i.e., “co-fed” with amolecular weight modifying agent).

SUMMARY OF THE INVENTION

[0011] We have found that improved floc ratings are shown for cationicacrylamide/quaternary ammonium salt copolymers when such copolymers areproduced by a process that requires continuous addition of thecross-linking agent to the polymerization reaction medium after about50% or more of monomer to polymer conversion has occurred, while noconcurrent addition of any chain transfer agent to the reaction mixtureis made. Additionally, it is preferred that during the polymerinitiation step, no cross-linking or chain transfer agent should bepresent.

[0012] Cationic acrylamide/quaternary ammonium salt copolymers made bythe process can be used in amounts of 1-2000 ppm, preferably 10-200 ppm,(based on one million parts of the aqueous medium) to improve dewateringof aqueous suspensions containing sludge, such as oily sludge.

DETAILED DESCRIPTION

[0013] This invention provides a high molecular weight, water-solublepolymer that exhibits improved flocculating performance. A process forpreparing the polymer is presented. In the process, a polymerizationreaction mixture is provided that comprises one or more acrylicmonomers. Polymerization of the acrylic monomers in the reaction mixtureis initiated in the absence of any chain branching or cross-linkingagents, or any chain transfer agents. A chain branching or cross-linkingagent is, however, added to the reaction mixture continuously, at theend of the polymerization reaction, preferably right before thepolymerization reaction is substantially completed. Typically, the chainbranching agent or cross-linking agent is a water-solublemultifunctional monomer having at least two unsaturated groups. In apreferred embodiment, N,N′-methylenebisacrylamide, also referred to asMBA, is used as the chain branching agent or cross-linking agent. TheMBA is added continuously after the polymerization reaction has achieveda total monomer conversion of about 50%-99%. Preferably, the MBA isadded after 75% monomer conversion. The polymerization reaction can beconducted in inverse emulsion, solution, or by a precipitationpolymerization process.

[0014] In accordance with the invention, it has been discovered thatcationic (meth) acrylamide/quaternary ammonium salt copolymers havingrepeat units (x) and (y) as shown in Formula I, prepared by the processdescribed herein, are effective in separating suspended solids fromaqueous media.

[0015] In Formula I above, the molar ratio of repeat units (x):(y) mayvary from 95:5 to 5:95 with the molar ratio (x):(y) of 60:40 beingpresently preferred. R₁ and R₂ are the same or different and are chosenfrom H and CH₃. Q is —C(O)O—, —OC(O)—, or —C(O)NH—, R₃ is branched orlinear (C1-C4) alkylene; R₄, R₅ and R₆ are independently chosen from H,C1-C4 linear or branched alkyl, or an C5-C8 aromatic or alkylaromaticgroup; A is an anion selected from Cl⁻, Br⁻, HSO₄ ⁻, or MeOSO₃ ⁻.

[0016] At present the preferred repeat units (y) are as follows:

[0017] 1—(AETAC)—2-acryloxyethyltrimethyl ammonium chloride; alsoreferred to as dimethylaminoethylacrylate methyl chloride; in terms ofFormula I above R₁=H; R₂=H; Q is —C(O)O—, R₃=Et; R₄, R₅ and R₆ are allMe and A is Cl—

[0018] 2—(MAPTAC)—3-(meth) acrylamidopropyltrimethyl ammonium chloride;in terms of Formula I above R₁=H; R₂=CH₃; Q is —C(O)NH—; R₃=Pr, R₄ R₅and R₆ are all Me and A is Cl⁻

[0019] 3—(METAC)—2-methacryloxyethyltrimethyl ammonium chloride; interms of Formula I above R₁=H; R₂=CH₃, Q is —C(O)O—; R₃ is Et and R₄, R₅and R₆ are all Me and A is Cl⁻

[0020] The presently preferred copolymer that is made by the methoddescribed herein is a 60:40 mole percent copolymer of acrylamide/AETACthat is cross-linked as explained hereinafter. The degree ofcross-linking is relatively minor and can amount from about 1×10⁻⁴% toabout 5×10⁻³% based upon 100 molar percent of the repeat units (x) and(y) present.

[0021] The resulting molecular weight of the resulting cross-linkedcopolymers may vary over a wide range, e.g., 10,000-20,000,000. Theinvention, however, finds its greatest usefulness when the acrylamidecopolymers have molecular weights in excess of 1,000,000. The copolymershould, however, be water-soluble.

[0022] The copolymers are prepared by a water-in-oil emulsion technique.Such processes have been disclosed in U.S. Pat. Nos. 3,284,393 and5,006,596 herein incorporated by reference. The technique comprises:

[0023] Preparation of an aqueous phase, typically ranging from about 50%to about 90% by weight of the total emulsion, which aqueous phase iscomprised of water, monomers as described above, chelating agents andinitiator(s), if the particular initiator(s) chosen are water-soluble.Ethylenediamine tetraacetic acid or diethylenetriamine pentaacetic acidand their salts are suitable, but not limiting, chelating agents. Thewater-soluble initiator may be selected from peroxides, persulfates, andbromates. Sulfites, bisulfites, sulfur dioxide, and other reducingagents used with oxidizing initiators to form an initiating redox pairmay also be used. If a reducing agent or a water-soluble azo-type,thermal initiator such as 2,2′-azobis-(2-amidinopropane)dihydrochloride, is used, it is added as described below. The totalamount of monomers will range from about 30% to about 80%, by weight,based on the total weight of the aqueous phase.

[0024] Preparation of an oil phase, ranging from about 10% to about 50%by weight of the total emulsion, which oil phase is comprised of aliquid organic hydrocarbon and water-in-oil emulsifying agents. Apreferred group of hydrocarbon liquids include aliphatic compounds. Oilscommonly used for this purpose are the hydrotreated petroleumdistillates, such as the commercially available materials sold under thetrademarks of Vista LPA-210, Shellsol D100S, and Exxsol D100S. The oilphase may optionally contain the initiator(s), if the particularinitiator(s) chosen are oil-soluble. Typical oil-soluble, thermalinitiators would be 2,2′-azo-bis (isobutyronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile) and benzoyl peroxide, and the like. It iswell known to those skilled in the art that the initiator(s) can bechosen to be either water- or oil-soluble depending on the particularneeds of the system.

[0025] The water-in-oil emulsifying agent is usually a low HLBsurfactant. Typical emulsifiers are mono and digylcerides, sorbitanfatty acid esters and lower N,N-dialkanol substituted fatty amides, andthe like, and are described in U.S. Pat. Re. No. 28,576.

[0026] A mixture of emulsifying surfactants, rather than singleemulsifier, may be preferred. The concentration of emulsifier can befrom about 3% to about 30% by weight, based on the total weight of theoil phase. Polymeric surfactants such as modified polyester surfactants(Hypermer, ICI) and maleic anhydride-substituted ethylene copolymers(PA-14 or 18, Chevron) may also be added to improve the mechanicalstability and increase the solids content of the emulsion.

[0027] After the aqueous phase and oil phase have been preparedseparately, the aqueous phase is then homogenized into the oil phase.Homogenizers, high shear pumps, or high-speed agitators that are capableof mixing the two phases into a homogeneous water-in-oil emulsion may beused. Any of the techniques to prepare the inverse emulsions well knownto those skilled in the art may be used. Typically, the particle size ofthe resulting emulsion is between 10 μm and 2 μm. After the emulsion isprepared, the system is then sparged with nitrogen to remove all oxygenfrom the system. The emulsion is under constant agitation orcirculation. Polymerization is then initiated by adding a reducing agentfrom a red-ox pair or by heat to induce the decomposition of a thermalinitiator in the emulsion after its addition. The temperature of thereaction medium is maintained at about 20° C. to about 75° C.,preferably about 35° C. to about 55° C.

[0028] As is stated elsewhere, during the initiation step, it ispreferred that this be conducted in the absence of a cross-linking agentor chain transfer agent. ¹³C NMR techniques can be utilized to assessthe degree of conversion of the monomers into the copolymer. Afterwaiting until about greater than 50% or more of the conversion hasoccurred, the cross-linking agent is then added continuously to thereaction mixture in the absence of addition of any chain transfer agent.Most preferably the cross-linking agent is added continuously after thepolymerization reaction has achieved a total monomer conversion of fromabout 75%-99%, more preferably 80%-95%.

[0029] As to the cross-linking agents that can be used, these are wellknown in the art and function to provide cross-linked polymers in whicha branch or branches from one polymer molecule are effectively linked orattached to other polymer molecules. The preferred cross-linker isN,N′-methylenebisacrylamide (MBA) but a host of other cross-linkingagents such as divinylbenzene, diethylene glycol diacrylate, propyleneglycol dimethacrylate, diallylfumarate, propylene glycol dimethacrylate,allylacrylate, diallylfumarate and vinylalkoxy silanes may also bementioned.

[0030] The cross-linking agent may be added in an amount of about 1 ppmto about 5×10⁻⁴ ppm based on the total amount of the reaction mixture.In a preferred embodiment, MBA is added in an amount of about 1-500 ppm,preferably from about 2 to about 150 ppm, more preferably from about 3to about 50 ppm and most preferably from about 4 to about 12 ppm basedon total monomers.

[0031] The polymers of the present invention should be added to anaqueous media containing suspended solids, for which improvedflocculation performance is desired, in an amount effective for thepurpose. In a preferred embodiment of the invention the aqueous media isa biologically activated sludge from a secondary wastewater treatmentfacility.

[0032] The method of the invention has shown particular promise incentrifugal separation techniques wherein the polymer is admitted to anaqueous suspension of oily sludge to improve dewatering of thesuspension in a centrifuge. The polymers made in accordance with theinvention can be employed in combination with conventional watertreatment additives such as organic and/or in inorganic coagulants,flocculants and anti-forming agents.

[0033] The invention will be described in conjunction with the followingexamples that should not be construed to limit the invention but shouldbe viewed for illustrative purposes only.

EXAMPLES Example 1

[0034] This example illustrates the synthesis of a 60:40 mol %acrylamide:dimethylaminoethylacrylate methyl chloride quaternary saltcopolymer in water-in-oil emulsion. To a suitable reaction flaskequipped with a condenser, a thermometer, a nitrogen inlet, and anoverhead mechanical agitator, were added 111.2 grams of Shellsol D100S,6.0 grams of sorbitan monooleate, and 4.6 grams of oleicisopropanolamide as steric stabilizers. The mixture was then stirredrapidly with a magnetic stirrer for 15 minutes. An aqueous phase wasprepared separately, which contained 140.7 grams of 53 wt % acrylamide(AM), 169.9 grams of 80 wt % dimethylaminoethylacrylate methyl chloridequaternary salt (AETAC), 31.9 grams of de-ionized water, 0.6 grams of 40wt % diethylenetramine pentaacetic acid pentasodium salt, and 1.8 gramsof 1 wt % potassium bromate. The solution was further adjusted with 1.0grams of 10 wt % sulfuric acid to pH 5.0. The aqueous phase mixture wasstirred rapidly with a magnetic stirrer for 15 minutes. The aqueousphase was then transferred to the flask containing the oil phase andboth phases were homogenized to a stable emulsion with an 8000-rpm,200-watt homogenizer for 30 seconds. The reactor was then configured foroperation under reflux conditions. The emulsion was agitated at 600 rpmand continuously sparged using 0.5 Liters per minute of nitrogen atambient temperatures for 50 minutes. Once the 50-minute nitrogen spargeperiod was complete, 0.3 grams of 0.7 wt % aqueous tertiary-butylhydroperoxide (t-BHP), an oxidizing agent, were added through a septumin the top of the sealed reactor. Mixing and sparging of the emulsioncontinued for an additional 10 minutes. The polymerization was theninitiated by slowly adding 7.0 grams of 0.5 wt % aqueous sodiummetabisulfite (SMBS), a reducing agent, to the reactor starting at arate of 0.055 milliliters per minute. A temperature increase of 2° C.(exotherm) indicated that the polymerization reaction had initiatedunder ambient conditions. The reaction temperature was allowed toincrease from 25° C. to 45° C. over a period of 15 minutes, and was thenmaintained at 48° C. for 2 hours. At the end of the 2-hour period thetotal monomer conversion was 84%, the polymerization temperaturedecreased by 1° C., and then 2.2 grams of 0.05 wt % aqueousN.N′-methylenebisacrylamide (MBA) was added continuously to the reactorat a rate of 0.111 milliliters per minute. After completion of theaddition of the MBA and 0.5 wt % SMBS feeds, 2.4 grams of 5 wt % aqueousSMBS were then added to the rector starting at a rate of 0.300milliliters per minute. After completion of the 5 wt % SMBS feed, 0.3grams of 0.7 wt % aqueous t-BHP were then added to the reactor. Thereaction mixture was stirred for 2 minutes and then 4.7 grams of 28 wt %SMBS was added to the reactor starting at a rate of 0.300 millilitersper minute. After cooling, 16.3 grams of a blend of ethoxylated C-12-14secondary alcohols and sulfosuccinic acid, 1,4-bis(2-ethylhexyl) ester,sodium salt, were added to the emulsion as inverting surfactants. Theresultant inverse-emulsion contained 42 wt % of active material. Thepolymer in aqueous solution had an intrinsic viscosity of 12.44 dL/g anda standard viscosity of 23.1 cps as measured in 1M NaCl at 25° C. usinga Brookfield Programmable DV-II+with a UL adapter at 12 RPM. Thesecharacteristics are summarized in Table 1.

Comparative Example 2—C-2

[0035] This example illustrates the formation of prior art 60:40 mol %acrylamide: dimethylaminoethylacrylate methyl chloride quaternary saltcopolymers at 42% active material in water-in-oil emulsion, in which themethylenebisacrylamide (MBA) is added to the polymerization reaction ina batch manner. This cationic emulsion polymer was obtained using theconditions of Example 1 with the MBA totally added to the aqueous phaseprior to homogenization and the start of the reaction. The MBA wasprepared in de-ionized water at a concentration of 500 ppm. An aliquotof this solution was added to the aqueous phase to prepare water-in-oilemulsion of 5 ppm of MBA. The intrinsic viscosity and standard viscosityare also listed in Table 1.

Comparative Example 3—C-3

[0036] This example illustrates the formation of prior art 60:40 mol %acrylamide: dimethylaminoethylacrylate methyl chloride quaternary saltcopolymers at 42% active material in water-in-oil emulsion, in whichmethylenebisacrylamide (MBA) is added to the polymerization reaction allat once as a single treatment. This cationic emulsion polymer wasobtained using the conditions of Example 1, except that a solution ofMBA is added the reaction mixture in one shot after 72% monomerconversion. The MBA was prepared in de-ionized water at a concentrationof 500 ppm. An aliquot of this solution was added to the reactionmixture to prepare water-in-oil emulsion of 5 ppm of MBA. The intrinsicviscosity and standard viscosity are also listed in Table 1.

Comparative Example 4—C-4

[0037] This example illustrates the formation of prior art 60:40 mol %acrylamide: dimethylaminoethylacrylate methyl chloride quaternary saltcopolymers at 42% active material in water-in-oil emulsion, in whichmethylenebisacrylamide (MBA) is added to the polymerization reaction ina continuous manner. This cationic emulsion polymer was obtained usingthe conditions of Example 1, except that a solution of MBA is added tothe reaction mixture continuously from the initiation of polymerizationuntil a total monomer conversion of 94%. The MBA was prepared inde-ionized water at a concentration of 500 ppm. An aliquot of thissolution was added to the reaction mixture to prepare water-in-oilemulsion of 5 ppm of MBA. The intrinsic viscosity and standard viscosityare also listed in Table 1.

Example 5—Ex-5

[0038] This example illustrates the formation of a 60:40 mol %acrylamide: dimethylaminoethylacrylate methyl chloride quaternary saltcopolymer at 42% active material in water-in-oil emulsion, in whichmethylenebisacrylamide (MBA) is added to the polymerization reaction ina continuous manner. This cationic emulsion polymer was obtained usingthe conditions of Example 1, except that a solution of MBA is added thereaction mixture in a continuous feed after 77% monomer conversion. TheMBA was prepared in de-ionized water at a concentration of 500 ppm. Analiquot of this solution was added to the reaction mixture to preparewater-in-oil emulsion of 5 ppm of MBA. The intrinsic viscosity andstandard viscosity are also listed in Table 1. TABLE 1 Monomer MBAStandard Intrinsic Weight % Conversion Feed Time Viscosity ViscosityCationic Charge Example (¹³C NMR) (minutes) (centipoides) (dL/gram)(isoelectric end pt) Ex 1 84%  20 min 23.1 12.44 50.76 C-2 0%  0 min 8.411.03 45.72 C-3 72%  0 min 12.5 11.70 49.16 C-4 0%-94% 155 min 14.212.92 44.85 Ex-5 77%  30 min 20.2 8.908 49.93

Example 6 Efficacy Results—LSCST

[0039] For evaluating the performance of the example polymers previouslydescribed as flocculants and dewatering agents, the following laboratoryscale centrifuge simulation test (LSCST) was developed and utilized:

[0040] 1. Untreated substrate was obtained from the feedstock of acentrifuge application.

[0041] 2. An acceptable polymer dosage was determined based on thepolymer treatment established for the application.

[0042] 3. The example emulsion polymer samples from the presentinvention were made down in de-ionized water to 0.3 wt % solids.

[0043] 4. A variable speed mixer capable of attaining a maximum speed of2000 RPM was set up with a 2-inch diameter turbine impeller.

[0044] 5. The mixer speed was set to 1600 RPM.

[0045] 6. A wide mouth plastic bottle was graduated for 200 ml.

[0046] 7. For each test, a 200-ml aliquot of untreated sludge was pouredinto the graduated plastic bottle. The prescribed amount of polymertreatment was then added to the untreated substrate.

[0047] 8. The bottle containing the treated mixture was placed below theimpeller, the mixer was then turned on, the bottle was raised tosubmerge the impeller in the mixture, and the mixture was stirred forthe designated amount of time, the bottle was lowered, and then themixer was turned off.

[0048] 9. The following guidelines were then used to quantitatively rankthe stability of the flocculated substrate.

[0049] Rating Guidelines

[0050] Floc size

[0051] 5=very large flocs (diameter>5 mm)

[0052] 4=good flocs (diameter 4-5 mm)

[0053] 3=medium flocs (diameter 2-3 mm)

[0054] 2=small flocs (diameter 1 mm)

[0055] 1=pin flocs (diameter<1 mm)

[0056] 0=no flocs

[0057] Table 2 summarizes the floc stability ranking for the polymers ofthe present invention relative to the other example polymers not of thisinvention (i.e., the C₂-C₄ comparative polymers). Untreated substratetypically does not form any floc after being stirred and therefore itsfloc size rating is 0 by definition. Addition of the polymers of thepresent invention to the substrate dramatically improved the flocquality after stirring as was evidenced by the maintained floc size overtime relative to the other treatments. All polymers tested were dosed at75 ppm. TABLE 2 5 Seconds 7 Seconds 10 Seconds 12 Seconds 15 SecondsExample Floc Rating Floc Rating Floc Rating Floc Rating Floc Rating Ex 14 3.5 3.5 3.5 2.5 C-2 2 1 0.5 0.5 0 C-3 3 3 3 3 2.0 C-4 3 2 1 0.5 0 Ex 53.5 3.5 3.5 3.0 2.5

[0058] As stated above, the cross-linked cationic, acrylamide/quaternaryammonium salt copolymers made by the processes set forth above are addedto an aqueous medium that includes organic matter therein, such as oilysludge. The copolymers may be admitted to any such media in an amount offrom about 1-2,000 ppm based upon one million parts of the medium.Preferably, the copolymer is admitted to such media in the amount ofabout 50-200 ppm. Use of the copolymers in such systems improves flocformation, thus enhancing separation of the floc from the aqueous phasevia conventional treatment techniques such as by flotation, beltpresses, centrifuges and the like.

[0059] In accordance with the patent statutes, the best mode ofpracticing the invention has been herein set forth. However, it will beapparent to those skilled in the art that modifications can be made inthe methods herein disclosed without departing from the spirit of theinvention. It is to be understood that the scope of the invention is tobe limited solely by the scope of the appended claims.

1. Improved method for preparing a cross-linked water soluble cationic(meth) acrylamide/quaternary ammonium salt copolymer that is useful as aflocculant, said method comprising 1) initiating polymerization of anaqueous reaction mixture comprising (meth) acrylamide monomers andquaternary ammonium salt monomers to convert said monomers into acopolymer containing reaction mixture comprising cationic poly (meth)acrylamide/quaternary ammonium salt copolymer; and 2) cross-linking saidcopolymer by addition of a cross-linking agent to said copolymercontaining reaction mixture; wherein said step of 2) cross-linkingcomprises waiting until about 50% or more of said monomers have beenconverted into said copolymer and then continuously adding saidcross-linking agent to said copolymer containing reaction mixture in theabsence of concurrent addition of any chain transfer agent to saidcopolymer containing reaction mixture.
 2. Improved method as recited inclaim 1 wherein in step 1) no chain branching or chain transfer agent isadded to say aqueous reaction mixture.
 3. Improved method as recited inclaim 1 wherein said cationic poly (methacrylamide)/quaternary ammoniumsalt copolymer has repeat units of the formula

wherein Q is —C(O)O—, —OC(O)—, or —C(O)NH—, R₃ is branched or linear(C₁-C₄) alkylene; R₄, R₅ and R₆ are independently chosen from H, C₁-C₄linear or branched alkyl, or an C₅-C₈ aromatic or alkylaromatic group; Ais an anion selected from Cl⁻, Br⁻, HSO₄ ⁻, or MeOSO₃ ⁻.
 4. Improvedmethod as recited in claim 3 wherein the molar ratio of repeat units(x):(y) is from about 80-20(x):20-80(y).
 5. Improved method as recitedin claim 4 wherein the molar ratio of repeat units (x):(y) is from about60-40(x):40-60(y).
 6. Improved method as recited in claim 3 wherein saidrepeat unit y is AETAC.
 7. Improved method as recited in claim 3 whereinsaid repeat unit y is MAPTAC.
 8. Improved method as recited in claim 3wherein said repeat unit y is METAC.
 9. Improved method as recited inclaim 1 wherein said copolymer is water-soluble and has a molecularweight of about 10,000-20,000,000.
 10. Improved method as recited inclaim 9 wherein said copolymer has a molecular weight of greater thanabout 1,000,000.
 11. Improved method as recited in claim 10 wherein saidcopolymer is acrylamide/AETAC and wherein the motor ratio of acrylamide:AETAC is about 60:40.
 12. Improved method as recited in claim 11 whereinsaid cross-linking agent is N,N′-methylenebisacrylamide.
 13. Method ofdewatering an aqueous suppression of the type including organic matterdispersed therein, said method comprising adding from about 1-2,000 ppmof a cross-linked water soluble cationic (meth)acrylamide/quaternaryammonium salt copolymer made in accordance with claim 1 to said aqueoussuspension, based upon one million parts of said suspension, formingflocs containing said organic matter and separating said flocs from saidaqueous system.
 14. Method as recited in claim 13 wherein said aqueoussuspension is contained within a centrifuge and said organic matter isoily sludge, said cross-linked water soluble cationic(meth)acrylamide/quaternary salt copolymer comprising a copolymerselected form the group consisting of acrylamide/AETAC;acrylamide/MAPTAC; and acrylamide/METAC copolymers and wherein saidseparating comprises centrifugal separation.
 15. Method as recited inclaim 14 wherein said water soluble cationic (meth)acrylamide/quaternarysalt copolymer is acrylamide/AETAC.